Timber Joists for Loft Conversion

27 Jan 2013
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United Kingdom
Dear all,

I’m doing a loft conversion and have a query about timber joists for the new loft floor.

I would like to design it like a continuous beam with one intermediate support and, in order to do so, I would need lengths of timber of about 6.3m.

The full specs for the floor joists would be: C24 treated and regularised timber, section 47x170, min length 6.3m.

Is this type of timber readily available from builder’s merchants? Or is it something that would need to be made to order and therefore more expensive / with longer lead times?

What is industry common practice with regard to the type and dimensions of timber being used?

Many thanks for your help,

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You can get non-standard lengths of timber no probs. I used some 7m long 47mm x 150mm fellas on a job in Staffs.
You are unlikely to find that length as a stock item unless you have a large timber merchants near to you so I would order it in advance.
As a side note you would not build like, that you would normally sister then together over the central supporting wall then 3.6m timber would do.
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@maxantax; is this because of a headroom issue?

Hi tony1851,

No, there is sufficient headroom.

It’s a bit difficult to explain without a drawing: I cannot have the steel directly under the dwarf wall because one end of the beam would have to rest on the chimney breast.

Once I move the beam to the nearest suitable location, I’m left with a long span and a point load where the dwarf wall is supported by the joists.

If I model the span as a simply supported beam I end up with a large section. If I use the continuous beam model, the bending moment mid span is obviously reduced and I can use a smaller section.

I was just wondering how easy it is to get hold of timber joists with a min length of 6.3m?

The other possibility would be to buy joists of “normal” length and then butt join them with splice plates. Is this current practice? Or would it be a messy job?

Many thanks for your replies.

Kind regards,

Would the dwarf wall really be that heavy as a point load on each joist?
Also, if the joists are a longer span so as to end up beyond the dwarf wall, then don't allow for any live load the other side of the dwarf wall.

In other words, the (longer) joist will not be carrying live load on its full span and therefore M max is reduced. I've used that approach several times and it can often work.

Alternatively, you can splice them - say over the support - but building control will probably look for calcs for the connections, and the moments induced by them :unsure:

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